Chapter 27 Quantum Physics. General Physics Quantum Physics II Sections 4–8.
2010_Kleppner_One Hundred Years of Quantum Physics
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One Hundred Years of Quantum PhysicsAuthor(s): Daniel Kleppner and Roman JackiwSource: Science, New Series, Vol. 289, No. 5481 (Aug. 11, 2000), pp. 893-898Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/3077316
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8/10/2019 2010_Kleppner_One Hundred Years of Quantum Physics
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P THW YS
O DISCOVERY
n e
undred
e a r s o f
Quantum
Physics
Daniel
Kleppner
and Roman
Jackiw
An informed
ist of the most
profound
cientific
developments
f the 20th
century
s
likely
o include
general
elativity,
uantum
mechanics,
ig bangcosmology,
he
unraveling
f the
genetic
ode,
evolu-
tionary
biology,
and
perhaps
few other
opics
of
the
reader's
hoice.
Among
these,
quantum
me-
chanics s
unique
becauseof
its
profoundly
adical
quality.Quantum
mechanics
orced
physicists
o
reshape
heir deas of
reality,
o rethink
he nature f
things
at the
deepest
evel,
and to revise heir
concepts
f
position
nd
peed,
as well as
theirnotions f cause
andeffect.
Although uantum
mechanicswas created
o describe n abstract tomicworld
arremoved rom
dailyexperience,
ts
impact
on our
daily
ives could
hardly
be
greater.
he
spectacular
dvances
n
chemistry, iology,
and medicine-and in
essentially very
other
science-could
not haveoccurredwithouthe tools thatquantummechanicsmadepossible.
Without
uantum
mechanicsherewouldbe no
globaleconomy
o
speak
of,
because
he electronics evolutionhat
brought
s the
computer
ge
is
a childof
quantum
mechanics. o is the
photonics
evolutionhat
brought
s the Information
ge.
The creation f
quantum hysics
has
transformedur
world,
bringing
with it all the benefits-and
therisks-of a scientific
evolution.'
Unlike
general elativity,
hich
grew
out of a brilliant
nsight
into the connection etween
gravity
and
geometry,
r the deci-
phering
f
DNA,
whichunveiled new worldof
biology,quan-
tummechanics
id not
spring
roma
singlestep.
Rather,
t was
created
n
one
of thoserareconcentrationsf
genius
hatoccur
from ime to
time in
history.
For 20
years
after heir ntroduc-
tion,
quantum
deaswere so
confused
hat herewas littlebasis
for
progress;
hen a small
group
of
physicists
reated
quantum
mechanics in three tumul-
tuous
years.
These scientists
o
"Quantum
were troubled
by
what
they
|:
were
doing
and n somecasesdis-
theory
is the
tressed
y
what
hey
haddone.
The
unique
ituation
f this
most
precisely
crucial
et
elusive
heory
s
per-
Papa
Quanta.
In
1900,
Max
t
haps
best summarized
y
the Planck tarted the
quantum-
|
ested and
most
following
observation:
Quan-
mechanicalnowball.
tum
heory
s themost
precisely
successful
tested ndmost uccessful
heory
n the
history
f science.Never-
theory
in
the
theless,
not
only
was
quantum
mechanics
eeplydisturbing
o its
theory
in
te
founders,
oday-75 years
after he
theory
was
essentially
ast
n
historyof
its currentorm-some of the luminariesf scienceremaindis-
satisfiedwith ts
foundationsnd ts
interpretation,
ven
as
they
-
8/10/2019 2010_Kleppner_One Hundred Years of Quantum Physics
3/7
PATHWAYS OF DISCOVERY
tially every
othermeasurable
roperty
f
matter,
uch
as
viscosity,elasticity,
lectrical nd hermal
onductivity,
o-
efficients of
expansion,
ndices of
refraction,
nd thermo-
elasticcoefficients.
Spurred y
the
energy
of
the
Victorian
work
ethic
and the
development
f
ever
more
ingenious
experimental
methods,
knowledge
accumulated t
a
prodigious
ate.
What is most
striking
to the contemporary ye,
however,
s
that the
com-
pendious descriptions
of
the
properties
of matter
were
essentially mpirical.
Thousands of
pages
of
spectral
data isted
precise
_ -
H
U
values for the
wavelengths
of the
elements,
but no-
_B
body
knew
why spectral _. :1
lines
occurred,
much less
what nformation
hey
con-
_
veyed.
Thermal ndelectri-
cal conductivitieswere in-
terpreted by suggestive
modelsthat fittedroughly
Superatom.
These
olorful
data
half of the
facts.
There measurementsf rubidiumtom
were numerous
empirical
ed Bose-Einsteinondensate.
laws,
but
hey
werenot sat-
isfying.
For
nstance,
he
Dulong-Petit
awestablished sim-
ple
relation etween
pecific
heatand he atomic
weight
of a
material.Muchof the time it
worked;
ometimes t didn't.
Themassesof
equal
volumesof
gas
were n the ratiosof in-
tegers-mostly.
The Periodic
Table,
which
provided
key
organizing rinciple
orthe
flourishing
cienceof
chemistry,
had
absolutely
o theoretical asis.
Among
he
greatest
chievementsf therevolutions this:
Quantum
mechanicshas
provided quantitativeheory
of
matter.
We
now understand
ssentially
very
detailof atomic
structure;
hePeriodic able
as
a
simple
and
natural
xplana-
tion;
and hevast
arrays
f
spectral
ata it intoan
elegant
he-
oretical ramework.
uantumheory
permits
he
quantitative
understanding
f
molecules,
f solidsand
iquids,
ndof con-
ductors nd semiconductors.t
explains
bizarre
henomena
suchas
superconductivity
nd
superfluidity,
ndexotic orms
of matter uchas the stuffof neutron
tars
nd
Bose-Einstein
condensates,
n whichall the
atoms
n
a
gas
behave ikea sin-
gle superatom. uantummechanics rovides ssential ools
forall of the
sciences nd or
every
advanced
echnology.
Quantum hysicsactually ncompasses
wo entities.The
first s the
theory
of matter t the atomic
evel:
quantum
me-
chanics. t is
quantum
mechanics hat allows us to under-
standand
manipulate
he materialworld.The
second
s the
quantumheory
of fields.
Quantum
ield
theoryplays
a to-
tallydifferent ole
n
science, o whichwe shallreturnater.
Quantum
Mechanics
The clue that
triggered
he
quantum
evolution
ame
not
from
studiesof
matter
ut
froma
problem
n
radiation. he
specific challenge
was to understandhe
spectrum
f
light
emitted
by
hot bodies:
blackbody
radiation. The
phe-
nomenon s familiar o
anyone
who has stared t a fire. Hot
matter
lows,
and he hotter
t
becomes he
brighter
t
glows.
The
spectrum
f
the
light
is
broad,
with
a
peak
that shifts
fromred
to
yellow
and
finally
o blue
(although
we
cannot
see
that)
as the
temperature
s raised.
l,fr
IS
C
It shouldhavebeen
possible
o understandhe
shape
of
the
spectrum y combining oncepts
rom
hermodynamic
and
electromagneticheory,
utall
attempts
ailed.
However,
by assuming
hat he
energies
f the
vibrating
lectrons hat
radiate he
light
are
quantized,
lanckobtained n
expres-
sion that
agreed
beautiful-
ly
with experiment.
ut as
he recognized
ll too well,
the theorywas physically
absurd, an act of desper-
ation," as he later de-
scribed t.
Planck applied his
quantum ypothesiso the
.?~ ,?.~~~~?energy
of the vibratorsn
the walls of a radiating
body. Quantumphysics
mighthave ended here f
in 1905 a novice-Albert
Einstein-had not reluc-
tantlyconcluded hat f a
vibrator's nergy s quan-
tized, then the energy of
the electromagneticield
romNIST n 1995, emerged from
that it radiates-light-
:oalescing
nto he firstdocument- must also be quantized.
Einstein thus imbued
light
with
particlelike
e-
havior,
notwithstanding
hatJamesClerkMaxwell's
heory,
and over a
century
of definitive
experiments,
estified to
light's
wave nature.
Experiments
n the
photoelectric
ffect
in the
following
decade evealedhatwhen
ight
s absorbed
its
energyactually
rrives
n
discrete
bundles,
as
if carried
by
a
particle.
he dualnature f
light-particlelike
r wave-
like
depending
n whatone looks for-was the firstexam-
ple
of a
vexing
heme
hatwould
recur
hroughoutuantum
physics.
The
duality
onstituted theoreticalonundrumor
thenext20
years.
The first
step
toward
quantumheory
had been
precipi-
tated
by
a dilemmaaboutradiation.The second
step
was
precipitated
y
a dilemma boutmatter. t was known hat
atoms contain
positively
and
negatively hargedparticles.
But
oppositely harged
articles
ttract.
According
o
elec-
tromagnetic heory,
herefore,
they should spiral into each
Atoms_~~ i; other, adiating
ight n a
broad
1913,_
esor r
spectrum
ntil hey
collapse.
Once again, the door
to
I
progress
was opened
by a
o
of_n~
ao
anovice:
Niels Bohr. In
1913,
_ ohr proposed a
radical
hy-
pothesis:Electrons n an atom
proble
aoisb
exist only
in certain
tationary
states, ncluding ground tate.
Electronschange their energy
dictions,
by "jumping"
etween the
ta-
.p..---
..-he tionary
states, emitting light
Atoms
go quantum. In whose wavelength ependson
1913,
Niels Bohrushered the energydifference. y com-
quantum hysicsnto
world
biningknown awswithbizarre
of atoms. assumptionsboutquantum e-
X
havior, Bohr swept away the
o
problem
f atomic tability. ohr's heorywas full of contra-
dictions,but it provideda quantitative escription f the
F
spectrum f the hydrogen tom.He recognized oth he suc-
11 AUGUST
000
VOL
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PATHWAYS OF DISCOVERY
cess and he
shortcomings
f his model.With
uncanny
ore-
sight,
he rallied
hysicists
o create new
physics.
His vision
was
eventually
ulfilled,
lthough
t took 12
years
anda new
generation
f
youngphysicists.
At
first,
attempts
o advanceBohr's
quantum
deas-the
so-called ld
quantumheory-suffered
one defeatafteran-
other.Then a series of
developments otally changed
he
courseof
thinking.
In
1923 Louis de
Broglie,
n
his Ph.D.
hesis,
proposed
that heparticle ehavior f lightshould
have ts
counterpart
n the wave
behav-
ior of
particles.
He associated wave-
length
with he
momentum f a
particle:
The
higher
he momentum he
shorter
the
wavelength.
he deawas
ntriguing,
but no one knewwhata
particle's
ave
nature
might ignify
or how t related o
atomic structure.
Nevertheless,
de
Broglie'shypothesis
was an
important
precursor
orevents oon o
take
place.
In the summerof
1924,
there was
yet
another
precursor.Satyendra
N.
Bose
proposed totally
new
way
to ex-
plain
he Planck adiationaw.He treat-
ed lightas if it were a gas of massless
particles now
called
photons)
hat do
not
obey
the classical laws of Boltz-
Getting
weirder.
mann
tatistics utbehave
according
o
said hat f
wavelike
a new
type
of
statistics asedon
parti-
like
particles,
hen
cles'
indistinguishable
ature.
Einstein
have
ikewaves.
immediately pplied
Bose's
reasoning
to a real
gas
of massive
particles
ndobtained new law-
to become known as the Bose-Einsteindistribution-for
how
energy
s shared
y
the
particles
n
a
gas.
Undernormal
circumstances,owever,
he new and old theories
predicted
the samebehavior or
atoms
n
a
gas.
Einstein ook no fur-
ther
nterest,
nd the result
ay undeveloped
or more
thana decade.
Still,
ts
key
idea,
he
ndistinguishability
of
particles,
was about
o
become
criticallymportant.
Suddenly,
tumultuous eries of eventsoccurred,
culminating
n
a scientific evolution.n the
3-yearpe-
riod rom
January
925 o
January
928:
*
Wolfgang
Pauli
proposed
he exclusion
principle,
providing
theoretical asis or he Periodic able.
*
Werner
Heisenberg,
with Max
Born
and
Pascual
Jordan,
iscovered
matrix
mechanics,
he first version
of
quantum
mechanics.The historical
goal
of
under-
standing
lectron
motionwithinatomswas abandoned
in favorof a
systematic
method
or
organizing
bserv-
able
spectral
ines.
*
Erwin
Schrddinger
nventedwave
mechanics,
a
second ormof
quantum
mechanics n which he state
of a
system
s described
y
a wave
function,
he solu-
tion to
Schrodinger'squation.
Matrixmechanicsand
wave
mechanics,
pparentlyncompatible,
ere shown
to be
equivalent.
Unl
*
Electronswereshown o
obey
a new
type
of statis-
arti
tical
law,
Fermi-Diractatistics. t was
recognized
hat
sorl
all
particles bey
eitherFermi-Dirac
tatistics r Bose-
ceri
Einstein
tatistics,
nd hat he two classeshavefunda-
mentally
ifferent
roperties.
*
Heisenberg
nunciatedhe
Uncertainty rinciple.
*
PaulA. M. Dirac
developed
a relativisticwave
equa-
tion for the electron hat
explained
lectron
pin
and
pre-
E
dicted
antimatter.
*
Dirac aid the foundations f
quantum
ield
theoryby
providing quantum escription
f the
electromagnetic
ield.
*
Bohr
announced he
complementarityprinciple,
a
philosophical rinciple
hat
helped
o resolve
apparent
ara-
doxesof
quantum
heory, articularly ave-particleuality.
The
principal layers
n the creation f
quantumheory
were
young.
In
1925,
Pauli was 25
years
old,
Heisenberg
and EnricoFermiwere
24,
and Diracand Jordanwere 23.
Schrodinger,
t
age
36,
was a late bloomer.
Born
and
Bohr
wereolder till,and t is significanthat heir
contributionswere
largely interpretative.
The
profoundly
adicalnatureof the intel-
lectual
achievement
s
revealed
y
Einstein's
reaction.
Having
nvented ome
of the
key
concepts
that
led to
quantum heory,
Ein-
stein
rejected
t. His
paper
on
Bose-Einstein
statisticswas his last contributiono
quan-
tum
physics
andhis last
significant
ontribu-
tion
to physics.
i~ '
^
~
Thata new
generation
f
physicists
was
needed to create
quantum
mechanics is
hardly surprising.
Lord Kelvin described
why in
a letter o Bohr
congratulating
im
on his 1913
paper
on
hydrogen.
He saidthat
therewas much ruth n Bohr's aper, uthe
would never understandt
himself. Kelvin
Louisde
Broglie
recognized
hat
radically
ew
physics
would
.
light
an
behave
need o come fromunfetteredminds.
particles
an
be-
In
1928,
the revolutionwas finished
and
the foundations f
quantum
mechanicswere
essentially
omplete.
The frenetic
pace
with
which it
occurred
s revealed
by
an anecdote ecounted
y
the late Abraham ais in Inward
Bound.
n
1925,
the con-
cept
of electron
spin
had been
proposed by
Samuel
Goudsmit nd
George
Uhlenbeck.
Bohrwas
deeplyskepti-
cal.
In
December,
e traveled o
Leiden,
he
Netherlands,
o
attend the jubilee of
Hendrik A. Lorentz's
d octrate. Pauli met
the traint
Hamburg,
Germany,
o
ind
out
Bohr's
pinion
a
bout
the
possibility
of
le
c-
tron pin.Bohrsaid
he
proposal was "very,
very interesting," his
well-known
put-down
phrase. Later at Lei-
den, Einstein
and Paul
Ehrenfest met Bohr's
train, also to discuss
spin. There, Bohr ex-
plained his
objection,
but Einstein showed a
way around t and con-
knowable eality.WernerHeisenberg vertedBohr nto a sup-
culated ne of the mostsocietally b- porter. On his
return
bed
deasof quantum hysics:he Un- journey,
Bohrmet with
tainty
rinciple.
yet more
discussants.
When the trainpassed
throughGdttingen,Germany,Heisenberg
nd Jordanwere
waitingat the station o ask his opinion.And at the
Berlin
station,Pauliwas waiting,having raveled specially rom
Hamburg.
ohr old them all that he
discovery
f electron
spin
was a
great
advance.
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